When a composition for external use such as a drug, a quasi-drug or a cosmetic product is applied to the skin or hair, it is required that the composition fits to the skin or hair with a fresh and refreshing feeling of use, which is characteristic of aqueous cosmetic materials such as a skin lotion or gel-like cosmetic materials. Besides, after application, it is required that the composition imparts wet moisture to the skin or hair with a smooth and soft feeling of use, which is characteristic of cosmetic materials containing an oily ingredient such as a cream or a milky lotion.
However, it is generally difficult to disperse oily ingredients and aqueous ingredients in a stable form together for a long term; thus it is difficult to achieve a fresh and refreshing feeling of use after having combined an oily ingredient with an aqueous cosmetic material, which can be used as a cream, a milky lotion or the like.
Emulsification is known as a technology for dispersing oily ingredients in an aqueous medium. In recent years, the emulsification technology has been evolving, and interest has been attracted to the creation of a new effect and feeling of use by emulsification. For example, it is expected to achieve a feeling of use free from an icky feeling of an oily ingredient by dispersing the oily ingredient in an aqueous composition in the form of fine emulsification particles. Attempts have been made to disperse an oily ingredient by converting it to fine emulsification particles under high shearing force in an aqueous medium (Okamoto T., et al., The 18th IFSCC international congress proceedings, Venezia, vol. 2, 327 (1994) (Non-Patent Document 1)). However, this method requires a special device to achieve the high shearing force.
Meanwhile, a whitening effect of a tranexamate ester has been known, and the same has been used as a whitening ingredient in compositions for external use. For example, Japanese Patent Laid-Open No. 04-46144 (Patent Document 1) describes an anti-pigmentation agent for external use using a tranexamate ester as an active ingredient.
Japanese Patent Laid-Open No. 2003-306419 (Patent Document 2) exemplifies tranexamic acid and derivatives thereof as whitening ingredients to be usable with coenzyme Q10. Japanese Patent Laid-Open No. 2004-107262 (Patent Document 3) exemplifies a cetyl tranexamate ester as an oil-soluble whitening agent to be usable with an L-ascorbic acid tetra-branched fatty acid ester derivative.
However, a problem of a tranexamate ester or a salt thereof is that it is sparingly soluble in water and oil and this is likely to cause aggregates in a formulation, and thus it is difficult to incorporate it in a formulation in a stable state.
For example, Japanese Patent Laid-Open No. 2002-234836 (Patent Document 4) describes an anti-stress composition for external use using a tranexamate ester as an active ingredient. It also describes dissolution of a tranexamate ester using a large amount of an oily ingredient such as olive oil. However, by this method, the formulation form is limited to a cream or a milky lotion due to the use of a large amount of an oily ingredient. Additionally, the obtained agent for external use is sticky and oily and the feeling of use is unfavorable.
Further, Japanese Patent Laid-Open No. 2006-306744 (Patent Document 5) describes a composition for external use using a tranexamate ester or a salt thereof as well as a silicone oil. It also states that by the combination of a tranexamate ester or a salt thereof and a silicone oil the solubility of the tranexamate ester or the salt thereof in the composition can be enhanced, and the effect of the tranexamate ester can last over an extended time period. Silicone oil is known as an oily ingredient giving a relatively refreshing feeling of use, but it is necessary to use a large amount thereof for dissolving a tranexamate ester, and such a fresh and refreshing feeling of use as given by aqueous cosmetic materials cannot be obtained.